The mechanism of action of the streptococcal cytolytic toxins, streptolysin O (SLO) and streptolysin S (SLS) on mammalian cells is being studied. The increased resistance to SLO observed in cell cultures which have been incubated in the presence of certain oxygenated derivatives of cholesterol is being investigated. These compounds inhibit cholesterol biosynthesis and reduce the content of cholesterol in the cell membrane; as a result the cells are rendered resistant to SLO. The alteration in cholesterol content of the plasma membrane which is necessary to make the cells resistant will be determined. In addition, the ability of resistant cells to recover their sensitivity to SLO following incubation in medium containing serum, cholesterol, or mevalonate will be examined.